Digital synchronous rectification control for flyback converter

ABSTRACT

A flyback converter includes a primary-side switch connected to a primary-side winding of a transformer and a secondary-side switch connected to a secondary-side winding of the transformer. The flyback converter is operated by controlling the primary-side switch to store energy in the transformer during ON periods of the primary-side switch, switching on the secondary-side switch synchronously with switching off the primary-side switch to transfer energy from the transformer to the secondary side, determining an off time of the secondary-side switch based on a reflected input voltage measured at the secondary-side winding when the primary-side switch is on, accounting for a settling time of the reflected input voltage when determining the off time of the secondary-side switch so that the settling time has little or no effect on the off time, and switching off the secondary-side switch based on the off time.

TECHNICAL FIELD

The present application relates to flyback converters, in particularsynchronous rectification control for flyback converters.

BACKGROUND

A flyback converter is a transformer-isolated converter based on thebasic buck-boost topology. In a flyback converter, a switch is connectedin series with the transformer primary side. The transformer is used tostore energy during ON periods of the primary switch, and providesisolation between the input voltage source and the output voltage. In asteady state of operation, when the primary switch is ON for a period ofTON, during the TON period, a diode on the secondary side becomesreverse-biased and the transformer behaves as an inductor. The value ofthis inductor is equal to the transformer primary magnetizing inductanceL_(M), and the stored magnetizing energy from the input voltage source.As such, the current in the primary transformer (magnetizing currentI_(M)) rises linearly from an initial value to a peak value. As thediode on the secondary side becomes reverse-biased, the load current issupplied from an output capacitor on the secondary side. The outputcapacitor value is ideally large enough to supply the load current forthe time period TON, with the maximum specified drop in output voltage.

To increase system efficiency, flyback converters typically useSynchronous Rectification (SR) controller and a secondary-side SR powerMOSFET. The secondary-side SR power MOSFET is turned on and offsynchronously with the primary side power MOSFET. Some conventionalsecondary-side controllers have an SR sense pin used for voltage sensingto turn off the SR power MOSFET on the secondary side, and which has avery high breakdown voltage requirement (e.g. up to 120V or evenhigher), so the chip technology used to implement the secondary-sidecontroller must support very high voltages. The SR sense pin is used forvoltage sensing, which has a very low negative threshold voltagecomparison requirement (e.g. around −10 mV with 10 uV accuracy), whichis very difficult to implement in standard chip technologies. Otherconventional secondary-side controllers do not require high voltagetechnology for the controller and do not need to compare against a verylow negative threshold voltage for detecting when to turn off thesecondary-side SR power MOSFET. However, these controllers suffer fromsettling time variation which causes the measurement of the reflectedinput voltage from the secondary side of the transformer to have someerror, especially for high frequency and high input line cases. Thisvariation greatly influences the calculation of the turn-on timing forthe secondary-side SR power MOSFET. Errors in the SR on-timecalculations is problematic, and leads to inefficient operation.Accordingly, conventional secondary-side controllers are designed forapplications operating over a relatively narrow operating range.Improved secondary-side controllers and SR control techniques aretherefore desired.

SUMMARY

According to an embodiment of a method of operating a flyback converterhaving a primary-side switch connected to a primary-side winding of atransformer and a secondary-side switch connected to a secondary-sidewinding of the transformer, the method comprises: controlling theprimary-side switch to store energy in the transformer during ON periodsof the primary-side switch; switching on the secondary-side switchsynchronously with switching off the primary-side switch to transferenergy from the transformer to the secondary side; determining an offtime of the secondary-side switch based on a reflected input voltagemeasured at the secondary-side winding when the primary-side switch ison; accounting for a settling time of the reflected input voltage whendetermining the off time of the secondary-side switch, so that thesettling time has little or no effect on the off time; and switching offthe secondary-side switch based on the off time.

According to an embodiment of a flyback converter, the flyback convertercomprises a primary-side switch connected to a primary-side winding of atransformer, a secondary-side switch connected to a secondary-sidewinding of the transformer, a primary-side controller operable tocontrol the primary-side switch to store energy in the transformerduring ON periods of the primary-side switch and a secondary-sidecontroller. The secondary-side controller is operable to: switch on thesecondary-side switch synchronously with switching off the primary-sideswitch to transfer energy from the transformer to the secondary side;determine an off time of the secondary-side switch based on a reflectedinput voltage measured at the secondary-side winding when theprimary-side switch is on; account for a settling time of the reflectedinput voltage when determining the off time of the secondary-sideswitch, so that the settling time has little or no effect on the offtime; and switch off the secondary-side switch based on the off time.

According to an embodiment of a secondary-side controller for a flybackconverter having a primary-side switch connected to a primary-sidewinding of a transformer and a secondary-side switch connected to asecondary-side winding of the transformer, the secondary-side controllercomprises circuitry operable to switch on the secondary-side switchsynchronously with switching off the primary-side switch to transferenergy from the transformer to the secondary side, determine an off timeof the secondary-side switch based on a reflected input voltage measuredat the secondary-side winding when the primary-side switch is on,account for a settling time of the reflected input voltage whendetermining the off time of the secondary-side switch, so that thesettling time has little or no effect on the off time, and switch offthe secondary-side switch based on the off time.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

The elements of the drawings are not necessarily to scale relative toeach other. Like reference numerals designate corresponding similarparts. The features of the various illustrated embodiments can becombined unless they exclude each other. Embodiments are depicted in thedrawings and are detailed in the description which follows.

FIG. 1 illustrates a block diagram of an embodiment of a flybackconverter with compensated secondary-side SR switch control.

FIG. 2 illustrates a waveform diagram of the compensated secondary-sideSR switch control technique.

FIG. 3 illustrate a circuit diagram of an analog-based embodiment of thecompensated secondary-side SR switch control technique.

FIGS. 4 through 6 illustrate different scenarios handled by thecompensated secondary-side SR switch control technique.

FIG. 7 illustrates a flow diagram of an embodiment of the compensatedsecondary-side SR switch control technique.

FIG. 8 illustrate a circuit diagram of a digital-based embodiment of thecompensated secondary-side SR switch control technique.

DETAILED DESCRIPTION

The embodiments described herein compensate for the settling timeportion of the primary-side switch of a flyback converter, and use thiscompensation to adjust the on-time period of the secondary-side SRswitch. In some embodiments, previous or present sampled input voltageinformation is used to modify the on-time period of the secondary-sideSR switch. In other embodiments, correct input voltage information isobtained from the previous switching cycle (settled voltage) and theturn-off time of the SR switch on the secondary side is optimized forthe next cycle based on this information. In general, the primary-sideswitch is controlled to store energy in the flyback transformer duringON periods of the primary-side switch. The secondary-side switch isswitched synchronously with switching off the primary-side switch totransfer energy from the transformer to the secondary side of theflyback converter. The off time of the secondary-side switch, whichoccurs at the end of the on-time period for the secondary-side switch,is determined based on the reflected input voltage measured at thesecondary-side winding of the flyback transformer when the primary-sideswitch is on. The settling time of the reflected input voltage isaccounted for when determining the off time of the secondary-sideswitch, so that the settling time has little or no effect on the offtime. The secondary-side switch is switched off based on the(compensated) off time.

FIG. 1 illustrates an embodiment of a flyback converter that includes aprimary-side switch Q1 connected to a primary-side winding Wp of atransformer 100, a secondary-side switch Q2 connected to asecondary-side winding Ws of the transformer 100 and a primary-sidecontroller 102 operable to control the primary-side switch Q1 to storeenergy in the transformer 100 during ON periods of the primary-sideswitch Q1. The primary-side and secondary-side switches Q1, Q2 are shownas power MOSFETs with integrated diodes in FIG. 1. However, any suitablepower transistors can be used for the primary-side and secondary-sideswitches Q1, Q2 such as but not limited to power MOSFETs, IGBTs(insulated gate bipolar transistors), HEMTs (high-electron mobilitytransistors), etc. Switching of the primary-side switch Q1 is controlledby the primary-side controller 102 which generates a signal Q1 _(CTRL)based on the input voltage Vin, current I_(Q1) through the primary-sideswitch Q1, and a zero-cross detect voltage developed by a resistordivider (not shown for ease of illustration) coupled to an auxiliarywinding (also not shown for ease of illustration) on the primary side ofthe flyback converter. Switching control of a primary-side switch of aflyback converter is well known in the art, and therefore no furtherexplanation is provided with respect to the switching control ofprimary-side switch Q1.

The flyback converter also includes a secondary-side controller 104 forcontrolling the secondary-side switch Q2 connected to the secondary-sidewinding Ws of the flyback transformer 100. The secondary-side controller104 switches on the secondary-side switch Q2 synchronously withswitching off the primary-side switch Q1, to transfer energy from thetransformer 100 to the secondary side of the flyback converter. Thesecondary-side controller 104 also determines the off time of thesecondary-side switch Q2, which occurs at the end of the on-time periodfor the secondary-side switch, based on the reflected input voltageV_(DET) measured at the secondary-side winding Ws of the transformer 100when the primary-side switch Q1 is on. The reflected input voltageV_(DET) measured on the secondary side has the correct information aboutthe bulk (input voltage) when the primary-side switch Q1 turns on, andis stepped down to voltage V_(PD) by a resistor divider formed byresistors R₁ and R₂ for input to the secondary-side controller 104.

The secondary-side controller 104 also accounts for the settling time ofthe reflected input voltage V_(DET) when determining the off time of thesecondary-side switch Q2, so that the settling time has little or noeffect on the off time, and switches off the secondary-side switch Q2based on the (compensated) off time.

When primary-side switch Q1 is turned off, secondary side peak currentbased on discontinuous conduction mode (DCM) operation is given by:

$\begin{matrix}{I_{SP} = {\frac{N_{P}}{N_{S}} \times I_{PP}}} & (1)\end{matrix}$where I_(SP) is the peak current of the secondary-side winding Ws,I_(PP) is the peak current of the primary-side winding Wp, N_(P) is theprimary winding turns, and N_(S) is the secondary winding turns. Thepeak current I_(PP) of the primary-side winding Wp and the peak currentI_(SP) of the secondary-side winding Ws are given by:

$\begin{matrix}{I_{PP} = {\frac{V_{in}}{L_{P}} \times T_{on}}} & (2) \\{and} & \; \\{I_{SP} = {\frac{V_{out}}{L_{S}} \times T_{DET}}} & (3)\end{matrix}$where L_(P) is the primary-side winding inductance, V_(in) is theprimary-side input voltage, V_(out) is the system output voltage, T_(on)is the turn-on period for the primary-side switch Q1, and T_(DET) is thetiming for secondary-side winding demagnetization, which should also bethe turn-on period T_(on) of the secondary-side switch Q2.

Inserting equations (2) and (3) into equation (1) yields:

$\begin{matrix}{{{\frac{V_{out}}{L_{S}} \times T_{DET}} = {\frac{N_{P}}{N_{S}} \times \frac{V_{in}}{L_{P}} \times T_{on}}},} & (4) \\{\frac{N_{P}}{N_{S}} = {\sqrt{\frac{L_{P}}{L_{S}}} = n}} & (5) \\{and} & \; \\{\frac{V_{in} \times T_{on}}{n} = {V_{out} \times T_{DET}}} & (6)\end{matrix}$From equation (6), the inductor average voltage is zero during aswitching period in steady state, so the product of charge-voltage andcharge-time is equal to the product of discharge-voltage anddischarge-time, which is referred to as the volt-second balanceequation.

Equation (6) can be used to predict a solution for the on-time periodand thus off time of the secondary-side switch Q2, but because ofreflected input measurement error and parasitic parameters in thesystem, equation (6) may not be followed closely. The secondary-sidecontroller 104 calculates the secondary-side winding demagnetizationtime T_(DET) for the previous switching cycle and the present product ofcharge-voltage and charge-time, cancelling errors introduced by themeasurement error and parasitic parameters.

For the n^(th) switching cycle, the charge balance equation is given by:

$\begin{matrix}{\frac{\left\lbrack {{V_{in}(n)} + ɛ} \right\rbrack \times {T_{on}(n)}}{n} = {{V_{out}(n)} \times {T_{DET}(n)}}} & (7)\end{matrix}$where ε is the measurement error for the reflected input voltageV_(DET). For the (n+1)^(th) switching cycle, the charge balance equationis given by:

$\begin{matrix}{\frac{\left\lbrack {{V_{in}\left( {n + 1} \right)} + ɛ} \right\rbrack \times {T_{on}\left( {n + 1} \right)}}{n} = {{V_{out}\left( {n + 1} \right)} \times {T_{DET}\left( {n + 1} \right)}}} & (8)\end{matrix}$Dividing equations (7) and (8) yields:

$\begin{matrix}{{T_{DET}\left( {n + 1} \right)} = {\frac{\frac{\left\lbrack {{V_{in}\left( {n + 1} \right)} + ɛ} \right\rbrack \times {T_{on}\left( {n + 1} \right)}}{n \times {V_{out}\left( {n + 1} \right)}}}{\frac{\left\lbrack {{V_{in}(n)} + ɛ} \right\rbrack \times {T_{on}(n)}}{n \times {V_{out}(n)}}} \times {T_{DET}(n)}}} & (9)\end{matrix}$Because the output capacitor C_(O) of the flyback converter isrelatively large, and for the two consecutive switching cycles (n) and(n+1), the output voltage will be the same and thereforeV_(out)(n)=V_(out)(n+1). Equation (9) can be simplified as given by:

$\begin{matrix}{{T_{DET}\left( {n + 1} \right)} = {\frac{\frac{\left\lbrack {{V_{in}\left( {n + 1} \right)} + ɛ} \right\rbrack \times {T_{on}\left( {n + 1} \right)}}{n}}{\frac{\left\lbrack {{V_{in}(n)} + ɛ} \right\rbrack \times {T_{on}(n)}}{n}} \times {T_{DET}(n)}}} & (10)\end{matrix}$

Based on the previous switching cycle demagnetization time and thecalculated turn-on period for the secondary-side switch Q2, thesecondary-side controller 104 updates the calculation and corrects thevolt-second equation to yield an accurate turn-on period and thus offtime for the secondary-side switch Q2 for the next switching cycle. Thesecondary-side controller 104 can tune the turn-on period and thus offtime for the secondary-side switch Q2 based on the difference betweenthe calculated turn-on time and the measured turn-on time, to yield moreaccurate results for turning on and turning off the secondary-sideswitch Q2.

FIG. 2 illustrates a waveform that shows how the secondary-sidecontroller 104 tunes the calculated turn-on period and turn off time forthe secondary-side switch Q2 as a function of the reflected inputvoltage V_(DET) measured at the secondary-side winding Ws of the flybacktransformer, where T_(onPrimary) is the on-time period of theprimary-side switch Q1, T_(SRmax) is the maximum allowed turn-on periodfor the secondary-side switch Q2, T_(SRon) is the compensated (adjusted)on-time period for switch Q2, and T_(dead) is the time for switch Q2 tobe turned off before approaching T_(SRmax).

FIG. 3 illustrates a block diagram of an analog-based implementation ofthe control technique implemented by the secondary-side controller 104.V-I (voltage-current) converter A₃ converts voltage V_(CD) into currentI_(DISCHR), where V_(CD) is the resistor divided output voltage providedby resistors R₃ and R₄ for input to the secondary-side controller 104.When the primary-side switch Q1 turns off, voltage V_(PD) goes negative.V-I converter A₃ discharges internal capacitor C_(T), which means thesecondary-side switch Q2 is turned on. V-I converter A₁ converts voltageV_(PD) into current I_(CHR), where V_(PD) is the resistor dividedreflected input voltage provided by resistors R₁ and R₂ and measured bythe secondary-side controller 104. V-I converter A₁ is on when theprimary-side switch Q1 is on and voltage V_(PD) is at a high voltage.

The secondary-side controller 104 uses the difference currentI_(CHR)−I_(DISCHR) to charge internal capacitor C_(T) from a defaultvoltage level V_(ref1) during the turn-on period of the primary-sideswitch Q1. Current I_(CHR) is given by:

$\begin{matrix}{I_{CHR} = \frac{\frac{R_{2}}{R_{1} + R_{2}} \times \left( {\frac{V_{in}}{n} + V_{out}} \right)}{R_{{int}\; 1}}} & (12)\end{matrix}$where R_(int1) is an internal resistor for converting voltage V_(PD)into current. Current I_(DISCHR) is given by:

$\begin{matrix}{I_{DISCHR} = \frac{\frac{R_{4}}{R_{3} + R_{4}} \times V_{out}}{R_{{int}\; 2}}} & (13)\end{matrix}$where R_(int2) is an internal resistor for converting voltage V_(CD)into current. From equations (12) and (13), the difference currentI_(CHR)−I_(DISCHR) is given by:

$\begin{matrix}{{I_{CHR} - I_{DISCHR}} = {\frac{\frac{R_{2}}{R_{1} + R_{2}} \times \left( {\frac{V_{in}}{n} + V_{out}} \right)}{R_{{int}\; 1}} - \frac{\frac{R_{4}}{R_{3} + R_{4}} \times V_{out}}{R_{{int}\; 2}}}} & (14)\end{matrix}$If R_(int1)=R_(int2) and

${\frac{R_{2}}{R_{1}} = \frac{R_{4}}{R_{3}}},$the difference current I_(CHR)−I_(DISCHR) becomes:

$\begin{matrix}{{I_{CHR} - I_{DISCHR}} = \frac{\frac{R_{5}}{R_{4} + R_{5}} \times \left( \frac{V_{in}}{n} \right)}{R_{{int}\; 1}}} & (15)\end{matrix}$From equation (15), the internal capacitor C_(T) is charged up only bythe reflected input voltage, not output voltage.

Comparator A₄ compares voltage V_(PD) with reference voltage V_(ref2).When voltage V_(PD) is lower than reference V_(ref2), one short pulse isgenerated by pulse generator A₂. This short pulse sets SR (set-reset)flip-flop A₅, and turns on the secondary-side switch Q2 through bufferblock A₅.

During the secondary-side winding demagnetization time, which should bethe turn-on period for the secondary-side switch Q2, the secondary-sidecontroller 104 uses current sink A₅ to discharge (I_(DISCHR)+I_(error))internal capacitor C_(T), where I_(error) is a programmable current thatcan be positive or negative. When the capacitor C_(T) voltage dischargesto V_(ref3) level, the secondary-side switch Q2 is turned off throughcomparator A₇, SR flip-flop A₅ and buffer A₆. SR flip-flop A₅ is resetvia logic block A₁₂ when both T_(SRmax) and the output of comparator A₇is positive. When the capacitor C_(T) voltage discharges to V_(ref1)level as indicated by comparator A₉, after some delay time T_(dead)provided by logic block A₁₃ and delay block A₁₀ to get t_(cal), timet_(cal) is compared with the falling edge t_(max) of T_(SRmax) and thesecondary-side controller 104 performs some judgement and adjustmentthrough logic block A₁₁.

For high system frequencies or high line (AC input) conditions, theturn-on period T_(ONPrimary) of the primary-side switch Q1 is reduced,and the reflected input voltage V_(PD) measured by the secondary-sidecontroller 104 is not accurate. Under these conditions, voltage V_(PD)settles. During the settling time, V-I converter A₁ is operating but notat the right level. As a result, V-I converter A₁ charges capacitorC_(T) to an improper level (the voltage information is not correctduring the settling time). When this voltage is converted to current forcharging capacitor C_(T), some error exists in I_(CHR). Ideally, voltageV_(PD) has no settling time and capacitor C_(T) is charged at the inputvoltage level Vin for the full on-time period (rectangular voltagesignal) of the secondary-side switch Q2. However, capacitor C_(T) may becharged lower than what it should ideally be charged to. This means thatthe on-time period and thus the off time for the secondary-side switchQ2 may be set shorter than what it ideally should be, lowering systemefficiency. From equation (6), the calculated turn-on period and offtime for the secondary-side switch Q2 may be adversely affected.

The secondary-side controller 104 corrects this error by compensatingfor the shape of voltage V_(PD), to account for the settling time sothat V_(PD) has a rectangular or quasi-rectangular shape. The circuitshown in FIG. 3 charges capacitor C_(T) and makes the voltage look likea rectangular or quasi-rectangular voltage signal, by compensating theV_(PD) signal. Pulse generator A₂, SR flip-flop A₅ and buffer block A₆sample and hold voltage V_(PD) at the proper level and thesecondary-side controller 104 uses this voltage to calculate the chargecurrent I_(CHR) for charging capacitor C_(T) during the next switchingcycle.

As shown in FIG. 2, the target turn-on period and thus turn off time forthe secondary-side switch Q2 should be given by:T _(SRon) =T _(SRmax) −T _(dead)  (11)where T_(SRmax) is the maximum allowed turn-on period for thesecondary-side switch Q2, and can be measured by a comparator includedin or associated with the secondary-side controller 104.

From equation (11), if T_(SRon) is shorter than T_(SRmax)−T_(dead), thesecondary-side controller 104 prolongs the turn-on period and thusdelays the turn off time of switch Q2 for the next switching cycle, andmakes the next switching cycle turn-on period equal toT_(SRmax)−T_(dead). If T_(SRon) is longer than T_(SRmax)−T_(dead), thesecondary-side controller 104 shortens the calculated turn-on period andthus pulls in the turn off time of switch Q2 for the next switchingcycle, and makes next switching cycle turn-on period equal toT_(SRmax)−T_(dead). If T_(SRon) is equal to T_(SRmax)−T_(dead), thesecondary-side controller 104 keeps the calculated turn-on period andthus off time of switch Q2 time for the next switching cycle, and makesthe next switching cycle turn-on period equal to T_(SRmax)−T_(dead).

FIGS. 4 through 6 illustrate the scenarios described above and handledby the secondary-side controller 104. Ideally, time t_(cal) shouldhappen at the same time as t_(max) for the present switching cycle. Iftime t_(cal) happens before time t_(max) as shown in FIG. 4, which meansthat the discharge time for capacitor C_(T) is too short for the presentswitching cycle, the secondary-side controller 104 reduces dischargecurrent I_(error) by ΔI so that t_(cal) approaches t_(max) for the nextswitching cycle.

If time t_(cal) happens at the same time as t_(max) as shown in FIG. 5,the discharge time for capacitor C_(T) is ideal for the presentswitching cycle. The secondary-side controller 104 maintains thedischarge current I_(DISCHR)+I_(error) at the present level, so thatt_(cal) will be at the same time as t_(max) for the next switchingcycle.

If time t_(cal) happens after t_(max) as shown in FIG. 6, t_(cal)happens after t_(max), which means that the discharge time for capacitorC_(T) is too long for the present switching cycle. The secondary-sidecontroller 104 increases the discharge current I_(error) e.g. by 5×ΔI,10×ΔI, or even higher so that t_(cal) will be earlier than t_(max) forthe next switching cycle.

FIG. 7 illustrates an embodiment of the control technique implemented bythe secondary-side controller 104. The control technique can be used forboth DCM and CCM (continuous conduction mode) operation of thesecondary-side switch Q2. The secondary-side controller 104 convertsvoltage V_(PD) into current I_(CHR) and voltage V_(CD) into currentI_(DISCHR) (Block 200). During the turn-on period of the primary-sideswitch Q1, the secondary-side controller 104 uses difference currentI_(CHR)−I_(DISCHR) to charge internal capacitor C_(T) (Block 202). Afterthe primary-side switch Q1 is turned off, the secondary-side controller104 turns on the secondary-side switch Q2 when V_(PD) is lower thanthreshold voltage V_(ref2) (Block 204). For the first switching pulse ofthe secondary-side switch Q2, the secondary-side controller 104 uses onecurrent I_(DISCHR) to discharge internal capacitor C_(T) during theturn-off period of the secondary-side switch Q2 (Block 206). Thesecondary-side controller 104 uses one programmable currentI_(DISCHR)+I_(error) to discharge internal capacitor C_(T) during theturn-on period of the secondary-side switch Q2 for the subsequentswitching cycles (Block 208). When the voltage across internal capacitorC_(T) is discharged to V_(ref3), the secondary-side controller 104 turnsoff the secondary-side switch Q2 (Block 210). Further, when the voltageacross internal capacitor C_(T) discharges to V_(ref1), with one fixeddelay time T_(dead) to get time t_(cal), which is compared with themaximum allowed T_(SRmax) falling edge t_(max) (Block 212), thesecondary-side controller 104 performs the following evaluations andadjustments (Block 214):

-   -   t_(cal) happens before t_(max), which means that discharge time        for capacitor C_(T) is too short for the present switching        cycle. For the optimal result, t_(cal) should happen the same        time as t_(max). For the next switching cycle, the        secondary-side controller 104 reduces discharge current        I_(error) by ΔI, and t_(cal) will be approach to t_(max) for the        next switching cycle;    -   t_(cal) happens the same time as t_(max), which means that        discharge time for capacitor C_(T) is already optimal for the        present switching cycle. The secondary-side controller 104        maintains the discharge current I_(error), and t_(cal) will be        the same as t_(max) for the next switching cycle;    -   t_(cal) happens after t_(max), which means that discharge time        for capacitor C_(T) is too long for the present switching cycle.        For the optimal result, t_(cal) should happen the same time as        t_(max). For the next switching cycle, the secondary-side        controller 104 increases creases the discharge current I_(error)        (for example, by 5×ΔI, 10×ΔI, or even higher), and t_(cal) will        be earlier than t_(max) for the next switching cycle.

With the control technique described above, the turn-on period T_(DET)and thus turn off time of the secondary-side switch Q2 can be calculatedbased on the previous switching cycle and the present product ofcharge-voltage and charge-time, and the errors induced by themeasurement of the reflected input voltage and parasitic parameters arecancelled. Also, the discharge current depends on output voltage andtherefore is adapted for a jump (sudden increase) in the output voltage.Furthermore, there is no need for high voltage chip technology with verylow threshold detection and the control technique can be used for bothDCM and CCM operation.

By using the volt-second balance equation, also based on the previousswitching cycle demagnetization time and present product of inputvoltage over turn-on time, the secondary-side controller 104 can correctthe volt-second equation and provide accurate transistor turn-on periodand thus off time for the next switching cycle. The turn-on period andoff time can be tuned based on the difference between calculated turn-ontime and measured turn-on time, to yield an optimum result. Errorsintroduced by the reflected input voltage measurement and parasiticparameters are also cancelled by the control technique described herein.

FIG. 8 illustrates a block diagram of a digital-based implementation ofthe SR control technique implemented by the secondary-side controller104. According to this embodiment, capacitor C_(T) is not used and thesecondary-side controller 104 does not necessarily need the previouscycle information. At the end of the present on-time period for theprimary-side switch Q1, the secondary-side controller 104 needs tosample the reflected input voltage measured at the secondary-sidewinding Ws. However, the secondary-side controller 104 does not know theend of the primary-side switch Q1 on-time period once the primary-sideswitch Q1 is turned on. In one embodiment, the secondary-side controller104 includes a sample-and-hold unit 300 which samples the stepped downversion V_(PD) of the reflected input voltage V_(DET) measured at thesecondary-side winding Ws over an open window that always samples, andcaptures the value when the primary-side switch Q1 turns off. Thefalling edge of the turn-off event of primary-side switch Q1 is detectedby an integrated capture compare unit (CCU) 302, which triggers thesample and hold unit 300 for the capturing event of the input voltage.Furthermore, the CCU 302 estimates the on-time period of theprimary-side switch Q1 based on the captured edges of the samplingwindow. The captured on-time is fed to a lookup table (LUT) 304 whichcontains the associated error figures for very small on-time values whenthe reflected voltage is not yet settled during primary-side switch Q1turn-on phase (see FIG. 2). The corresponding error figure is fed to acalculation unit 306 for determining the on-time of the secondary-sideswitch Q2, which is based on the volt-second theorem equation previouslydescribed herein. By considering the error figure from the LUT 304within the calculation for the on-time of the secondary-side switch Q2,the sampled deviated input voltage is compensated for very small on-timeperiods of the primary-side switch Q1. The secondary-side switch Q2turns on immediately when the primary-side switch Q1 turns off. Thesecondary-side controller 104 provides a minimum on-time period for thesecondary-side switch Q2, and calculates the off-time during thisperiod.

Terms such as “first”, “second”, and the like, are used to describevarious elements, regions, sections, etc. and are also not intended tobe limiting. Like terms refer to like elements throughout thedescription.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise.

It is to be understood that the features of the various embodimentsdescribed herein may be combined with each other, unless specificallynoted otherwise.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A method of operating a flyback converter havinga primary-side switch connected to a primary-side winding of atransformer and a secondary-side switch connected to a secondary-sidewinding of the transformer, the method comprising: controlling theprimary-side switch to store energy in the transformer during ON periodsof the primary-side switch; switching on the secondary-side switchsynchronously with switching off the primary-side switch to transferenergy from the transformer to the secondary side; determining an offtime of the secondary-side switch based on a reflected input voltagemeasured at the secondary-side winding when the primary-side switch ison; accounting for a settling time of the reflected input voltage whendetermining the off time of the secondary-side switch, so that thesettling time has little or no effect on the off time; and switching offthe secondary-side switch based on the off time, wherein accounting forthe settling time of the reflected input voltage when determining theoff time of the secondary-side switch comprises: sampling values of thereflected input voltage during a previous switching cycle of thesecondary-side switch and holding one of the values sampled atsteady-state of the reflected input voltage; and using the value of thereflected input voltage sampled at steady-state during the previousswitching cycle of the secondary-side switch in determining the off timeof the secondary-side switch for a present switching cycle of thesecondary-side switch.
 2. A method of operating a flyback converterhaving a primary-side switch connected to a primary-side winding of atransformer and a secondary-side switch connected to a secondary-sidewinding of the transformer, the method comprising: controlling theprimary-side switch to store energy in the transformer during ON periodsof the primary-side switch; switching on the secondary-side switchsynchronously with switching off the primary-side switch to transferenergy from the transformer to the secondary side; determining an offtime of the secondary-side switch based on a reflected input voltagemeasured at the secondary-side winding when the primary-side switch ison; accounting for a settling time of the reflected input voltage whendetermining the off time of the secondary-side switch, so that thesettling time has little or no effect on the off time; and switching offthe secondary-side switch based on the off time, wherein accounting forthe settling time of the reflected input voltage when determining theoff time of the secondary-side switch comprises: sampling values of thereflected input voltage during a present switching cycle of thesecondary-side switch and holding one of the values sampled atsteady-state of the reflected input voltage; and using the value of thereflected input voltage sampled at steady-state during the presentswitching cycle of the secondary-side switch in determining the off timeof the secondary-side switch for the present switching cycle.
 3. Themethod of claim 2, wherein using the value of the reflected inputvoltage sampled at steady-state during the present switching cycle ofthe secondary-side switch in determining the off time of thesecondary-side switch for the present switching cycle comprises: usingan estimated on-time of the primary-side switch as a lookup value in atable to identify one or more error signals used to yield the off timeof the secondary-side switch for the present switching cycle.
 4. Aflyback converter, comprising: a primary-side switch connected to aprimary-side winding of a transformer; a secondary-side switch connectedto a secondary-side winding of the transformer; a primary-sidecontroller operable to control the primary-side switch to store energyin the transformer during ON periods of the primary-side switch; and asecondary-side controller operable to: switch on the secondary-sideswitch synchronously with switching off the primary-side switch totransfer energy from the transformer to the secondary side; determine anoff time of the secondary-side switch based on a reflected input voltagemeasured at the secondary-side winding when the primary-side switch ison; account for a settling time of the reflected input voltage whendetermining the off time of the secondary-side switch, so that thesettling time has little or no effect on the off time; switch off thesecondary-side switch based on the off time; sample values of thereflected input voltage during a previous switching cycle of thesecondary-side switch and hold one of the values sampled at steady-stateof the reflected input voltage; and use the value of the reflected inputvoltage sampled at steady-state during the previous switching cycle ofthe secondary-side switch in determining the off time of thesecondary-side switch for a present switching cycle of thesecondary-side switch.
 5. A flyback converter, comprising: aprimary-side switch connected to a primary-side winding of atransformer; a secondary-side switch connected to a secondary-sidewinding of the transformer; a primary-side controller operable tocontrol the primary-side switch to store energy in the transformerduring ON periods of the primary-side switch; and a secondary-sidecontroller operable to: switch on the secondary-side switchsynchronously with switching off the primary-side switch to transferenergy from the transformer to the secondary side; determine an off timeof the secondary-side switch based on a reflected input voltage measuredat the secondary-side winding when the primary-side switch is on;account for a settling time of the reflected input voltage whendetermining the off time of the secondary-side switch, so that thesettling time has little or no effect on the off time; switch off thesecondary-side switch based on the off time; sample values of thereflected input voltage during a present switching cycle of thesecondary-side switch and hold one of the values sampled at steady-stateof the reflected input voltage; and use the value of the reflected inputvoltage sampled at steady-state during the present switching cycle ofthe secondary-side switch in determining the off time of thesecondary-side switch for the present switching cycle.
 6. The flybackconverter of claim 5, wherein the secondary-side controller is operableto use an estimated on-time of the primary-side switch as a lookup valuein a table to identify one or more error signals used to yield the offtime of the secondary-side switch for the present switching cycle.